For example, if you photographed a Ciel and Sebastian (Black Butler) before, would you photograph these characters again if a different cosplayer asked you to? Would you only do this if it's a special circumstance (close friend, etc)?

When I did this, I realized I didn't have as many ideas the second time around, and I wasn't as excited about taking the photos.

This is a good question. I basically shoot what inspires me. This directly relates to your question because if I know I already have good images of a Tifa or a Babydoll then I won't feel inspired to find and shoot another. That being said, though I may be less inclined to shoot those cosplays again if I AM inspired to shoot, then I'll put in just as much effort if not more as I did the first time around. This doesn't only apply to costumes either, it applies to cosplayers as well. So if I took photos of a person on day 1 of a convention, then I know I'll have a more strict threshold of whether or not I'll work with them again during the same convention. Sometimes it can even carry on for multiple years.

It's never personal, it's just a matter of wanting to experiment and capture new things,